Can I lose 50 lbs and stay motivated?

I've lost ten so far. and i do believe that i can lose 50 more, but i get really down on myself if i don't lose enough weight for the week and then i overeat. how do i stop myself from doing that? also, after i lose like 20 pounds i start slacking on my diet and exercise cuz i feel good so i'm like well i can cheat. idk why i have that mentality. how do i get past it?